Coun"ter*bore` (-b?r`), n. 1. A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.

2. A kind of pin drill with the cutting edge or edges normal to the axis; -- used for enlarging a hole, or for forming a flat-bottomed recess at its mouth.

Coun`ter*bore" (koun`t?r-b?r"), v. t. To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.

• a cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface
  
• to create such a conical recess